It is difficult to measure air data (e.g., wind speed and direction) on a ducted fan air-vehicle, such as an unmanned aerial vehicle (UAV). The difficulty is due in part to the air speed of the ducted fan air-vehicle. The air speed of a ducted fan air-vehicle can be much slower than a typical fixed-wing aircraft. In fact, ducted fan air-vehicles have the ability of low speed flight and are well known for stationary hovering aerodynamic performance. These low air speeds make it difficult to measure air data with typical air data systems, as most air data sensors are not sensitive enough at low speeds to obtain reliable air data.
Air data systems typically use pressure sensors to make air data measurements. For example, air data systems generally implement static and pilot tube pressure sensors in order to derive altitude of the air-vehicle, wind velocity, and/or vehicle velocity. Pilot tubes are generally effective at providing air-speed and altitude information by measuring static pressure, dynamic pressure, and temperature about an air-vehicle. However, pilot tubes have limited value for hovering and low speed flight because they lose accuracy at speeds below about 20 m/s. Therefore, pilot tubes have limited value when measuring airflow surrounding a ducted fan air-vehicle in low speed and hovering flight, such as airflow caused by wind gusts and wind shears.
Further, it is difficult to place pilot tubes on an aircraft such that the pilot tubes are capable of measuring airflow from all directions. Pilot tubes are used to place pressure sensors outside the disturbance caused by a vehicle traveling through the air. In a ducted fan hovering condition these pilot tubes would have to extend multiple duct diameters in all directions to reach out to undisturbed air. Pilot tubes can extend out from a fixed wing air-vehicle up to seven inches.
Static pressure sensors can be flush mounted to the frame of an aircraft, however, as with pilot tubes, air data systems employing flush mounted static pressure sensors similarly lose accuracy at speeds below about 30.5 m/s. Other methods capable of measuring air data parameters exist, such as optical data systems. However, such optical data systems may be too large to be used on many ducted fan air-vehicles, such as a UAV or a Miniature Air Vehicle (MAV).
Another difficulty in measuring air data surrounding a ducted fan air-vehicle is that the ducted fan itself generates airflow around the air-vehicle. The thrust of a ducted fan creates its own generated environment around the air-vehicle, and this environment can disturb the airflow several diameters around the air-vehicle. A significant amount of downward expended air re-circulates back up around the outside of the air-vehicle and is returned back to the duct inlet. This generated air environment makes obtaining cross-wind components of airflow difficult. Moreover, this effect of the generated air environment diminishes the ability of traditional static and pilot tube air data systems to measure airflow disturbances outside the generated environment. As a result, the air data system may have difficulty in compensating for these disturbances.
The purpose in measuring surrounding airflow is for controlling stability or contributing to the control of air-vehicle flight. An avionics system may be used to control the altitude, positioning, and forward speeds of a ducted fan air-vehicle. The avionics system may benefit from the use of sensor inputs from an air data system in order to control the air-vehicle.
Generally, holding a fixed position in space of an air-vehicle relative to ground objects is dependent on inertial or GPS sensors. However, such sensors react to displacement caused by the disturbances of wind gusts and wind shears. Since the vehicle is moved from a fixed position to generate a control error for position, the airflow may result in significant movement of an air-vehicle before compensation by a vehicle control system utilizing inertial or GPS sensors.
Therefore, it would be beneficial to provide a ducted fan air-vehicle with an air data system for improved vehicle control that can accurately measure at hovering and low speed flight conditions the speed, direction, and the resulting displacement forces on the vehicle from cross winds, wind shears, wind gusts, and time dependent aerodynamic forces.